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(NBC News: Danielle Leigh) - The Medicare Chief, who over saw the botched debut of the Affordable Care Act website, publicly apologized Tuesday.
Marilyn Tavenner was the first Obama administration official to testify before Congress since the roll-out.
After hours of questioning, it was clear lawmakers are frustrated and not too ready to buy the administration's promise that the site is well on its way toward being fixed.
Tavenner started her testimony with an apology.
"This initial experience has not lived up to our expectations or the expectations of the American people and it is not acceptable."
Her statement was meant to ease frustration over the healthcare website, but lawmakers weren't backing down.
"Can't imagine how you think the system is working," said republican Congressman Vern Buchanan of Florida.
"How do you not know how many people have enrolled," asked republican Congressman Dave Camp of Michigan.
Tavenner refused to say how many people have actually enrolled, but she repeated promises the website is improving and the administration would have an update by mid-November.
"The bottom-line conclusion is this healthcare.gov site is fixable," said Tavenner.
Democrats tried to change the focus.
"If everybody would pitch in to make it work the goals that have been set would get more readily met.," said democratic Congressman Sandy Levin of Michigan.
The White House also addressed reports that millions of Americans who get coverage individually will lose their current insurance next year despite promises from President Obama.
"The fact is millions of Americans subject to the vagaries of this insurance market are going to have stronger coverage," said White House Spokesman Jay Carney.
For the administration, it was another day defending the president's signature law.
Health and Human Services Secretary Kathleen Sebelius will likely face similar questions when she testifies Wednesday.